weak_ptr is a non-owning observer of an object managed by shared_ptr. It doesn't increment the reference count, so the object can be destroyed even if weak_ptrs exist. Use weak_ptr to break reference cycles and implement observer patterns without keeping objects alive.

Use weak_ptr to break cycles in shared_ptr graphs, particularly for parent-child or observer-subject relationships.

This example demonstrates using weak_ptr to break a reference cycle between nodes.

// compile: g++ -o weak weak.cpp
// run: ./weak
// description: weak_ptr breaks reference cycles between objects
 
#include <iostream>
#include <memory>
 
class Node {
public:
    Node(int val) : value(val) { std::cout << "Node " << value << " created\n"; }
    ~Node() { std::cout << "Node " << value << " destroyed\n"; }
 
    int value;
    std::shared_ptr<Node> next;
    std::weak_ptr<Node> prev;  // weak to prevent cycle
};
 
int main() {
    {
        auto a = std::make_shared<Node>(1);
        auto b = std::make_shared<Node>(2);
 
        a->next = b;
        b->prev = a;  // weak reference, doesn't keep a alive
 
        if (auto p = b->prev.lock()) {
            std::cout << "Prev node value: " << p->value << "\n";
        }
    }  // Both nodes destroyed here
 
    return 0;
}